Silver Nanowire Electrodes Research Articles

Extremely flexible, transparent, and strain-sensitive electroluminescent device based on ZnS:Cu-polyvinyl butyral composite and silver nanowires

A multifunctional alternate current electroluminescent device (ACEL) was achieved by compositing ZnS:Cu particles in polyvinyl butyral (PVB) with two layers of percolated silver nanowire (AgNW) electrodes. The strong hydrogen bonding interactions and entanglement of PVB chains considerably strengthened the PVB, and thus, the cured mixture of ZnS:Cu particles and freestanding PVB required no additional support. The device was fabricated by embedding AgNWs on both sides of the ZnS:Cu-PVB composite film using an inverted layer process and intense-pulsed-light treatment. The strong affinity of PVB to the polyvinyl pyrrolidone (PVP) layer, which capped the AgNWs, mechanically stabilized the device to such an extent that it could resist 10,000 bending cycles under a curvature radius of 500μm. Using AgNW networks in both the top and bottom electrodes made a double-sided light-emitting device that could be applied to wearable lightings or flexible digital signage. The capacitance formed in the device sensitively varied with the applied bending and unfolding, thus demonstrating that the device can also be used as a deformation sensor.

Chemical sintering of direct-written silver nanowire flexible electrodes under room temperature

Transparent and flexible electrodes on cost effective plastic substrates for wearable electronics have attract great attention recently. Due to the conductivity and flexibility in network form, metal nanowire is regarded as one of the most promising candidates for flexible electrode fabrication. Prior to application, low temperature joining of nanowire processes are required to reduce the resistance of electrodes and simultaneously maintain the dimensionality and uniformity of those nanowires. In the present work, we presented an innovative, robust and cost effective method to minimize the heat effect to plastic substrate and silver nanowires which allows silver nanowire electrodes been directly written on polycarbonate substrate and sintered by different electrolyte solutions at room temperature or near. It has been rigorously demonstrated that the resistance of silver nanowire electrodes has been reduced by 90% after chemical sintering at room temperature due to the joining of silver nanowires at junction areas. After ∼1000 bending cycles, the measured resistance of silver nanowire electrode was stable during both up-bending and down-bending states. The changes of silver nanowires after sintering were characterized using x-ray photoelectron spectroscopy and transmission electron microscopy and a sintering mechanism was proposed and validated. This direct-written silver nanowire electrode with good performance has broad applications in flexible electronics fabrication and packaging.

Cold Isostatic-Pressured Silver Nanowire Electrodes for Flexible Organic Solar Cells via Room-Temperature Processes.

Transparent conducting electrodes (TCEs) are considered to be an essential structural component of flexible organic solar cells (FOSCs). Silver nanowire (AgNW) electrodes are widely used as TCEs owing to their excellent electrical and optical properties. The fabrication of AgNW electrodes has faced challenges in terms of forming large uniform interconnected networks so that high conductivity and reproducibility can be achieved. In this study, a simple method for creating an intimate contact between AgNWs that uses cold isostatic pressing (CIP) is demonstrated. This method increases the conductivity of the AgNW electrodes, which enables the fabrication of high-efficiency inverted FOSCs that have a power conversion efficiency of 8.75% on flexible polyethylene terephthalate with no short circuiting occurring as the CIP process minimizes the surface roughness of the AgNW electrode. This allows to achieve 100% manufacturing yield of FOSCs. Furthermore, these highly efficient FOSCs are proven to only be 2.4% less efficient even for an extreme bending radius of R ≈ 1.5 mm, compared with initial efficiency.

Degradation mechanisms of silver nanowire electrodes under ultraviolet irradiation and heat treatment

We report the degradation mechanisms of the silver nanowire (Ag NW) electrodes that play a significantly important role in the stability of wearable and flexible devices. The degradation mechanisms behind the increase in the sheet resistances of Ag NW electrodes were clarified by investigating the variations in the structure and the chemical composition of the Ag NW electrodes caused by ultraviolet irradiation and thermal treatment. While the shapes of the Ag NWs were affected by melting during the thermal degradation process, the chemical composition of the polyvinylpyrrolidone protective layer on the surfaces of the Ag NWs was not changed. Ultraviolet irradiation deformed the shapes of the Ag NWs because nitrogen or oxygen atoms were introduced to the silver atoms on the surfaces of the Ag NWs. A graphene-oxide flake was coated on the Ag NW electrodes by using a simple dipping method to prevent ultraviolet irradiation and ozone contact with the surfaces of the Ag NWs, and the increase in the sheet resistance in the graphene-oxide-treated Ag NWs was suppressed. These observations will be of assistance to researchers trying to find novel ways to improve the stability of the Ag NW electrodes in next-generation wearable devices.

Failure criterion of silver nanowire electrodes on a polymer substrate for highly flexible devices

Nanomechanical characteristics of standalone silver nanowires (Ag NWs) are a key issue for providing a failure criterion of advanced flexible electrodes that are trending towards smaller radius of curvatures (ROCs). Through in-situ tensile and buckling tests of pentagonal Ag NWs, we demonstrated that the intrinsic fracture strain provides a significant criterion to predict the mechanical and electrical failure of Ag NW electrodes under various strain modes, because the decrease in fracture strain limits figure of merit of flexible devices. The Ag NW electrodes on a polymer substrate exhibited a strain-dependent electrical failure owing to the unique deformation characteristics with a size-dependent brittle-to-ductile transition of the five-fold twinned Ag NWs. All the Ag NWs greater than approximately 40 nm in diameter exhibited brittle fracture with a size-independent stress-strain response under tensile and buckling modes, which leads to the electrical failure of flexible electrodes at the almost same threshold ROC. Meanwhile, the higher ductility of Ag NWs less than 40 nm in diameter resulted in much smaller threshold ROCs of the electrodes due to the highly extended fracture strains, which can afford a high degree of freedom for highly flexible devices.

Electroluminescent nanocellulose paper

Surface light-emitting transparent nanocellulose papers operating by alternating current voltages were demonstrated, where phosphors were embedded in the paper and silver nanowire electrodes were formed on both side of the paper. Silver nanowire electrodes were first coated on membrane filter with vacuum filtration using wet wiper tissues and then successfully transferred to the paper surface by a mechanical compression. The power consumption of a light-emitting paper was small in less than 0.36W/cm2. We could control the luminescent color and intensity from light-emitting papers by adjusting electrical frequency and the applied voltage level. White light emission was also demonstrated by using phosphors with a broad luminescence spectrum. The results develop into a versatile approach for commercializing the nanofibrillated cellulose and pave the way towards the realization of commercially viable, flexible and transparent light-emitting papers.

Mixed-dimension silver nanowires for solution-processed, flexible, transparent and conducting electrodes with improved optical and physical properties

In this work, we present an alternative method for the spray coating of silver nanowire contact electrodes by employing a mixture of short and long nanowires. Mixed silver nanowires are found to give improved optical properties with 2–5% higher transparency for the same sheet resistance of 25 Ω sq−1 compared to silver nanowires prepared with a single geometry. The figure of merit for the 25 Ω sq−1 sheet resistance electrode is found to be highest for the mixed-composition AgNWs-M1 (mixture of AgNWs-30L and AgNWs-60S) electrodes. The average root mean square surface roughness (Rq) parameter by white light interferometry is found to be lower for mixed-composition silver nanowire electrodes (Rq = 3–4 nm) compared to individual parent fixed-dimension-type silver nanowire electrodes (Rq = 6–8 nm).

Reduced yellowing of silver nanowire transparent conductive electrodes by simple hydrazine treatment

This paper introduces a simple hydrazine treatment method to reduce the yellowing of silver nanowire transparent conductive electrodes. To investigate the effects of hydrazine treatment on the yellowing of silver nanowire electrodes, the optical and electrical properties of silver nanowire electrodes were analyzed before and after hydrazine treatment. The optimal hydrazine treatment conditions were investigated by varying the concentration and thickness of hydrazine; the effect of hydrazine treatment on the long-term stability of silver nanowires was also analyzed.

Bending fatigue behavior of silver nanowire networks with different densities

This study examines the deformation behavior of silver nanowire networks with different densities under bending fatigue testing conditions. Up to 400,000 bending cycles are applied to the various silver nanowire networks while monitoring the resistance in situ. The least dense silver nanowire network shows a moderate decrease in resistance during the early bending cycles due to mechanical welding, followed by a rapid increase as the bending test progresses. As the nanowire density increases, however, this initial decrease in the resistance becomes smaller, and the resistance of the densest silver nanowire network increases sharply from the very start of the cyclic bending. A microstructural analysis reveals that the junction formation by mechanical welding in the early stages of bending is pronounced for the less dense silver nanowire networks, while the densest network showed severe deformation, including broken or disintegrated nanowires due to their confined geometry, which cannot easily accommodate the bending strain by stretching the network. Our findings underscore that different criteria should be considered to evaluate the mechanical reliability of various flexible electronic devices using silver nanowire electrodes with different densities.

Enhanced conductivity of transparent and flexible silver nanowire electrodes fabricated by a solution-processed method at room temperature

A simple method was developed to increase the electrical conductivity of Ag nanowire (NW) transparent electrodes prepared at room temperature. By dipping Ag NW coated poly(ethylene terephthalate) (PET) film into AgBr solution with sodium ascorbate as reductant, Ag nanoparticles grow on the surface of Ag NWs and their junction positions. It was found that the attachment of Ag nanoparticles on Ag NWs decreases the electrode film resistance from 220–300Ω/sq. to 30–50Ω/sq., which is mainly due to the change in junction resistance with Ag nanoparticles functioning as a bridge for electron transportation between Ag NWs. Ag NW coated PET films have a transmittance of 85.0% at the spectral wavelength of 550nm, and this value does not change much after AgBr treatment. A mechanical study shows that Ag NW electrodes on flexible substrates have excellent robustness subjected to bending. The properties of transparent Ag NW electrodes prepared using our method meet the requirements of transparent electrodes for many applications and could be a replacement for indium tin oxide for flexible electronics and solar cells.

Simple silver nanowire patterning using a DUV lamp direct write with sol–gel IZO capping

We develop a method to pattern silver nanowire (AgNW) electrode by using DUV lamp to directly write on indium–zinc-oxide capped AgNW. The patterned electrodes keep good conducting property on flexible polyimide substrate after 1000-times bending.

Effect of Synthesis Temperature, UV-Ozone Treatment, and Nanowire Diameter on the Failure of Silver Nanowire Electrodes

Silver nanowires (SNWs) have shown much promise as a lower cost flexible transparent electrode alternative to indium tin oxide. However, the stability of SNW electrodes has been brought into question. In this work, we studied the failure of SNW electrodes for different synthesis conditions and nanowire diameters with and without ultraviolet (UV)-ozone treatment. Transparent electrodes fabricated from nanowires synthesized at lower temperatures were found to have a lifetime of 53 days: the longest reported lifetime to the best of our knowledge for electrodes fabricated with nanowires less than 100 nm in diameter. In trials for transparent electrodes fabricated with SNW with average diameters of 80 nm, UV-ozone-treated films reached ultimate failure two days later than nontreated electrodes. This was attributed to the formation of silver oxide on the nanowires, which served as a protective layer and slowed the degradation process. Scanning electron microscope (SEM) images showed SNW electrodes made with an average nanowire diameter of 80 nm failed due to broken connections at the nanowire junctions. On the other hand, SNW electrodes made with an average diameter of 233 nm failed along the nanowires themselves, which suggests a different failure mechanism. The UV-ozone-treated larger diameter nanowire electrodes failed after 127 days, and the failure date of the nontreated SNW electrode remained undetermined after 153 days of continuous operation.

Soft imprinted Ag nanowire hybrid electrodes on silicon heterojunction solar cells

We demonstrate enhanced efficiencies in front-contacted silicon heterojunction (SHJ) solar cells using silver nanowire-based hybrid electrodes. SHJ cells typically suffer from shading losses due to reflection from macroscopic sun-facing metal fingers, which must be closely spaced to avoid resistive losses in the transparent conductive electrode (TCE). Using substrate conformal imprint lithography (SCIL) we fabricate silver nanowire electrodes on practical scale (4.0 cm2) planar SHJ cells. These electrodes exhibit anomalous transmission and a 7-fold improvement in sheet conductance relative to a standard ITO layer, enabling larger finger spacings and reducing reflection losses without compromising the cell fill factor. Over 70% of the ITO is replaced with transparent SiNx, reducing the use of indium while improving the anti-reflective performance. Combined, the reduced shading and reflection raises the short circuit current density increases by 2.1 mA cm−2, yielding an absolute increase in cell efficiency of 1.0%. These engineered hybrid electrodes provide a practical pathway towards front-contacted SHJ cells with a reduced dependence on rare metals and high efficiencies.

Mechanically Robust Silver Nanowires Network for Triboelectric Nanogenerators

The authors develop a mechanically robust silver nanowires (AgNWs) electrode platform for use in flexible and stretchable triboelectric nanogenerators (TENGs). The embedding of an AgNWs network into a photocurable or thermocurable polymeric matrix dramatically enhances the mechanical robustness of the flexible and stretchable TENG electrodes while maintaining a highly efficient triboelectric performance. The AgNWs/polymeric matrix electrode is fabricated in four steps: (i) the AgNWs networks are formed on a hydrophobic glass substrate; (ii) a laminating photocurable or thermocurable prepolymer film is applied to the developed AgNWs network; (iii) the polymeric matrix is crosslinked by UV exposure or thermal treatment; and (iv) the AgNWs‐embedded polymeric matrix is delaminated from the glass substrate. The AgNWs‐embedded polymeric matrix electrodes with four different sheet resistances, controlled by varying the AgNWs network deposition density, are deployed in TENG devices. The authors find that the potential difference between the two contact surfaces of the AgNWs network‐embedded polymer matrix electrodes and the nylon (or perfluoroalkoxy alkane) governs the output triboelectric performances of the devices, rather than the sheet resistance. Both Kelvin probe force microscopy and numerical simulations strongly support these observations.

(Invited) Intrinsically-Stretchable, Transparent Thin Film Transistors

Thin-film field effect transistor (TFT) is a fundamental component behind morden electronic devices including displays, sensor arrays, microprocessers, and identification tags. Here, we report the development of new conductor, semiconductor, and dielectric materials that exhibit elastomeric deformability and solution processability. A silver nanowire based screen printing ik was formulaed to print silver nanowire electrodes with conductivity as high as 47K S/cm which are embedded in an elastomer matrix to preserve the electrical conductivity at large strains. A semiconductive single-walled carbon nanotube network was employed as the channel material, and a polyurethane-co-polyethylene oxide as elastomeric dielectric. Intrinsically stretchable TFTs were fabricated using these new electronic materials. The fabrication processes involved the preparation of the source, drain and gate electrodes, semiconductor layer, dielectric layer, and substrate by solution-based techniques under ambient conditions. The resulting devices exhibited a mobility of ~30 cm2V-1s-1, on/off ratio of 103-104, switching current >100 mA, transconducance >50 mS, and relatively low operating voltages. The devices could be stretched by up to 50% strain while retaining a high mobility >10 cm2V-1s-1. No significant loss in electrical property was observed after 500 cycles of repeated stretching to 20% strain. The TFTs were used to drive organic light-emitting diodes with brightness controllable from 0 cd/m2 (off) to 200 cd/m2 (a typical display brightness). The approach and results represent an important progress toward the development of stretchable active matrix displays and other stretchable electronic devices.

Integration of optical and electrochemical sensors on a microfluidic platform using organic optoelectronic components and silver nanowires.

Since the emergence of microfluidic platforms sensors integration has been a major challenge. With the advances in miniaturization of these platforms, there is a need for solutions to integrate various optical components in order to build sensors, which will offer different detection characteristics such as several emission and sensing wavelengths. Moreover, the integration of an electrochemical sensor including a transparent electrode that will be compatible with the optical sensor represents an additional challenge. In this perspective, organic optoelectronic devices combined with silver nanowire electrodes could be a solution. The integration of a fluorescent sensor and an electrochemical oxygen sensor into a microfluidic platform and the different characteristics, advantages and disadvantages that offer organic light-emitting diodes (OLED), organic photodetectors (OPD) and silver nanowire electrodes are discussed. Finally, an example of the integration of an optical and an electrochemical sensor into a microfluidic chip for water pollution detection will be described.

Automation Highlights from the Literature

Automation Highlights from the Literature

Extremely Robust and Patternable Electrodes for Copy-Paper-Based Electronics

We propose a fabrication process for extremely robust and easily patternable silver nanowire (AgNW) electrodes on paper. Using an auxiliary donor layer and a simple laminating process, AgNWs can be easily transferred to copy paper as well as various other substrates using a dry process. Intercalating a polymeric binder between the AgNWs and the substrate through a simple printing technique enhances adhesion, not only guaranteeing high foldability of the electrodes, but also facilitating selective patterning of the AgNWs. Using the proposed process, extremely crease-tolerant electronics based on copy paper can be fabricated, such as a printed circuit board for a 7-segment display, portable heater, and capacitive touch sensor, demonstrating the applicability of the AgNWs-based electrodes to paper electronics.

Performance improvement in flexible polymer solar cells based on modified silver nanowire electrode

In this work, an efficient flexible polymer solar cell was achieved by controlling the UV-ozone treatment time of silver nanowires (Ag NWs) used in the electrode and combined with other modification materials. Through optimizing the time of UV-ozone treatment, it is shown that Ag NWs electrode treated by UV-ozone for 10 s improves the power conversion efficiency (PCE) of the device based on the blend of poly(3-hexylthiophene)(P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) from 0.76% to 1.34%. After treatment by UV-ozone, Ag NWs electrodes exhibit several promising characteristics, including high optical transparency, low sheet resistance and superior surface work function. As a consequence, the performance of devices utilizing 10 s UV-ozone-treated Ag NWs with PEDOT:PSS or MoO3 as composite anode showed higher PCEs of 2.77% (2.73%) compared with that for Ag NW electrodes without UV-ozone treatment. In addition, a PCE of 5.97% in flexible polymer solar cells based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl](PBDTTT-EFT):[6, 6]-phenyl C71-butyric acid methyl ester (PC71BM) as a photoactive layer was obtained.

Ageing of Silicone-Based Dielectric Elastomers Prepared with Varying Stoichiometric Imbalance: Changes in Network Structure, Mechanical, and Electrical Properties

Silicone‐based dielectric elastomers are promising dielectric electroactive polymers (DEAPs) applicable to various actuator applications. However, the lack of information concerning their long‐term performance still limits their industrial use. Here, the time‐dependent behavior of silicon‐based DEAPs under electromechanical cycling is investigated. A series of thin silicone films prepared with different stoichiometric imbalances are coated with compliant silver nanowire electrodes and then electromechanically cycled under alternating voltage (Vpp = 2 kV, VDC = 1 kV) over 105 cycles. Afterward, changes in cross‐linking density, elastic modulus, permittivity, and breakdown behavior are examined. With increasing load cycles, electrically induced cross‐linking of the hydrosilane groups occurs, which leads to increased cross‐linking density of the material. Concomitantly, increase in elastic modulus and decrease in permittivity are observed, resulting in a significant deterioration of actuation performance. The measured breakdown strength, which is related to an extrinsic breakdown strength based on electromechanical instability, increases too.image